Methanotrophic activity in relation to primary and bacterial production in a boreal humic lake

摘要

Microbial decomposition of organic matter causes hypoxic and anoxic conditions in the hypolimnion of polyhumic lakes during summer and winter stratification periods. The concentration of CH_4, an end product of anaerobic decomposition of organic matter, may as high as 250 mmol m~(-3) in the hypolimnion of these lakes (Huttunen et al. 2002, Huotari Kankaala unpubl.). A large proportion of the CH_4 produced is oxidized to CO_2 by methanotrophs or converted to microbial biomass (Rudd Hamilton 1974, Bastviken et al. 2003), while the rest is degassed into the atmosphere mainly during the spring and autumn turnover periods (Riera et al. 1999). Stable carbon isotope (δ~(13)C) composition of zoo-plankton has given rise to a hypothesis that methanotrophs may be a significant source of energy and carbon in the food webs of humic lakes (Jones et al. 1999), where the algal primary production is insufficient to support the productivity of zooplankton (Ojala Salonen 2001). In 2002 we studied CH_4 oxidation in relation to primary and bacterial production in the water column of a boreal, polyhumic Lake Valkea-Kotinen. The lake has been intensively monitored since 1990 by Finnish Environment Institute and Lammi Biological Station (see Keskitalo et al. 1998).